The main requirements for current inlet conductors in the field of vacuum lamps are the following:
The current inlet conductor passed through a vitreous material closing device is generally to be connected to an element made of any metal in the innerspace of the device. The connection should be characterized by a defined level of mechanical and electrical features. In the case of electric light sources the electric inlet conductor is usually connected by welding, particularly by resistance welding to the metallic element. The principal requirement is consequently high weldability. Another requirement can be drafted as follows: the current inlet conductor should be capable of being surrounded and wetted by a vitreous material in order to produce a hermetic seal. The current inlet conductors of the vacuum devices are most generally made of molybdenum. In vacuum devices, for example, such as high-pressure gas discharge lamps or halogenide lamps comprising a bulb made of quartz glass, thin plates or foils made of molybdenum are used for supplying electric current to the electrodes. In the case of light sources with bulbs made of hard glass or in electronic tubes, molybdenum wires are generally used. In order to facilitate welding of the molybdenum parts and to improve the quality of binding to elements made of molybdenum, the molybdenum element characterized by low weldability is covered with an intermediate metal coating. Another possibility lies in applying between the surfaces to be connected to one another, an element made of intermediate metal of required characteristics. The material of the intermediate metal coating or element is chosen in order to ensure required mechanical features such as strength and toughness at higher temperatures of operation. These requirements especially in the case of halogenic lamps or lamps with metallic halogenic filling, are to resistance against the atmosphere surrounding the metallic element. According to the art the more widely used intermediate metals are tantalum and platinum, however, in the literature it is mentioned that the use of zirconium, rhodium, nickel, gold or palladium and the alloys thereof can be advantageous (e.g. W. Espe: "Material of High Vacuum Technology", Vol. 1., Ch. 9, Pergamon Press, 1966).
When applying a special intermediate point made of a required metal, it is obvious that a further element should be taken into account during production in that the metal has to be supplied at the place of operation and positioned according to the requirements. The process is thereby made more complex than without the coating step. It is an obvious simplifying step to cover the elements made of molybdenum with a coating of intermediate metal at least on the surface parts where welding is to be later applied. The coating should be prepared before applying the element for production of the light source. The French Patent Specification No. 2 079 541 proposes the previous covering of the molybdenum foil by lubricating the surface of the foil with a solution of a platinum alloy which can be fixed by following with heat treatment.
The published West German Patent Specification No. DE-OS 31 04 043 discloses a method of preparing the coating on the surface of a molybdenum foil by vacuum sputtering or evaporation of a suitable metal. This specification discloses also the use of platinum, and further of tantalum, gold and rhenium. These metals are mentioned as to be theoretically suitable for preparing a coating. The vacuum sputtered or evaporated coating has a thickness in range from 20 to 10,000 nm.
There are no more patent publications known of by the Applicant wherein the use of rhenium as a material for intermediate metal elements or coating is disclosed.
The main requirements for the intermediate metal are the facilitating of the welding and making possible a vacuum-tight seal in the vitreous material applied in the vacuum device in which a molybdenum element as a current inlet wire is provided. Taking into account the fact that for practicing the technology it is required to make a coating along the whole surface of the element made of molybdenum in a continuous method, it is obvious that the coating is prepared as a continuous layer. Therein lies a further requirement in that the coating should be made of a material which can be moistened or wetted by the vitreous material when it is in a melt or plastic state, because if it cannot be moistened, a reliable vacuum-tight binding is impossible between the glass and the intermediate metal coating. The noble metals, i.e. gold and platinum are not suitable for satisfying the mentioned requirements and as the Applicant experienced, the current inlet made of molybdenum and covered with a noble metal can not be vacuum sealed, by the glass, during operation the vacuum device losses the vacuum tightness of the closing. The same can appear during storage. The requirement of moistening can be satisfied by tantalum, however it is disadvantageous in that the element with a coating made of tantalum cannot be treated further in a protecting atmosphere comprising hydrogen and if it is done, the current inlet wire with tantalum coating is rendered unsuitable for further use. Such heat treatment, however, could be advantageous for reaching at least two aims:
1. The heat treatment is capable of strengthening the binding between the basic molybdenum part and the intermediate metal coating by means of diffusion.
2. By heat treatment it is possible to improve the surface layer of the current inlet wire according to the requirements of a glass-metal interface. From experience it is well known that the molybdenum foil applied to flat parts of a quartz glass vessel should be built in immediately or quickly after heat treatment because after some days of storage in air, the reliability of the junction realised thereby falls in an expressive manner. When welding is carried out in unsufficient circumstances, it can occur that the metal surface is subjected to oxidation and a unit comprising the current inlet wire and the objects welded to it should be annealed once more in hydrogen comprising environment i.e. in a reducing atmosphere.